In my system at the moment, ratings between 0 and 30 (or 0 and 1 1/2 stars) represent varying degrees of dislike. 40 means "eh". 50-60 (2 1/2 to 3 stars) means I like the song but it's nothing special. 70-100 represent varying degrees of really liking the song. "-1" just means I haven't rated the song yet.
I rate based on the song, the performance, and the quality of the recording. A good song performed badly but recorded well might well get a 40 or 50 rating. I tend to rate on the bell curve: lots of 50s and 60s, fewer extreme numbers. There aren't a lot of real low rated songs. I'm less likely to add a crummy song to my collection than a good song, so the ratings are skewed to the upper half of the range.
I've been tweaking my rating system lately, so some songs' ratings might be off a little based on the description I just gave. My goal is get within 10 points of my "true" rating of the track. In other words, if I rate the same song on separate occasions, I'd like my new rating to be within 10 points (1/2 star) of the old rating. If I can't be consistent, the ratings aren't worth much!
If this all sounds more than a little OC, I plead partially guilty. I've had a lot of practice assigning personal ratings to songs when I was involved with LaunchCast (Yahoo's old music service) some years ago. With LaunchCast, the listener was rewarded for rating songs, the reward being more "better" songs being played for you in the future, based on what you like. After a little while, rating songs during a listening session became second nature, so it's easy to do it now for the songs in my own collection.
Let me know what you think. Helpful or not?